Two or more semiconductor chips and associated circuits may be assembled together in a single package, referred to as a hybrid or multi-chip module (MCM). The package serves as a protective container for the semiconductor chips, ancillary electrical components, and associated electronic circuits, that are typically supported on a common base. The base generally supports, or consists directly of, a printed circuit element which includes the electrical interconnection paths for the various components and associated connections to the outside of the package.
In order to provide an hermetic seal for the package a lid is generally attached by an appropriate metallurgical bond around the perimeter such that the various components are enclosed within. A seal ring is typically incorporated in the package base to facilitate forming the metallugical bond to the lid. This seal ring may take various forms such as metal side walls on a metal package, a metallized ring surrounding the enclosed component area on a ceramic package, or a separate raised metal ring metallurgically bonded to a ceramic package.
A desirable feature of MCM packages is that they be "reworkable", in the event one or more of the semiconductor devices in the MCM fail, and require replacement. Whereas an individually packaged semiconductor chip cannot be repaired and can simply be discarded if it fails, it is not economical to discard an entire MCM if one of its many semiconductor chips fails. Thus it is desirable that the package be sealed in such a manner that it is possible to remove the lid, remove and replace the failed device(s), and thereafter replace the lid and reseal the package.
In certain instances benefits such as size, weight, and power reductions, and improved electrical performance may be realized by increasing the overall size of MCM packages to envelope more circuit elements that would otherwise be in separate packages. As the length or width dimensions of the associated large MCM lids are driven beyond about 2 inches, forces on the lids can become problematic.
Significant forces on large hermetic MCM lids arise as a result of differential pressure between the inside and outside of the package. This occurs during post-seal leak testing when an external overpressure is used to drive a detectable leak tagging gas into the package and later when high altitude or space applications reduce the external ambient pressure to levels far below the gas pressure entrapped in the enclosed package.
The effect of these large forces is a tendency for the lid to deflect inward, possibly contacting and damaging enclosed components, or outward where undesireable contact with structure or other parts located in close proximity may occur. In addition to the possibility of undesireable contact, the deflection creates large forces on the metallurgical bond between the lid and seal ring, enough to exceed elastic stress limits, with an associated tendency to lose hermeticity.
It may be shown by approximating a section of the lid with beam equations that lid deflection is proportional to PL.sup.4 /t.sup.3, where P is differential pressure, L is the unsupported beam length, and t is the beam thickness. From this it may be appreciated that lid deflection for a 2.5 inch span is approximately 2.6 times that of a 2.0 inch span. It may also be recognized that to minimize deflection simply by increasing lid thickness, the lid thickness must increase geometrically relative to the increase in span. For large MCMs, this leads to prohibitively heavy lids, especially for avionics and spacecraft applications.
Other solutions have been offered to protect internal components from inward deflection, and the seal from large stresses, but each solution poses an offsetting disadvantage. Internal support posts or other intermediate internal supports require space within the package impacting the area available for component placement and routing.
Others have recently identified and publicized the foregoing problems in an article entitled "Lid Deflection for MCMs in Airborne Application, Huey et al., ICEMCM '96 Proceedings of the 1996 EHM Conference on Multichip Modules, Apr. 7, 1996, pp. 167-172, which the reader is invited to review. In that article, the authors recognize the foregoing solutions and explore the disadvantages of those solutions at greater length than here. The solution the article's authors think best is to hermetically seal the MCM at a lower gas pressure of one-half atmosphere, instead of the normal one atmosphere pressure. The present invention also seeks to ensure continued integrity of the lid in such high pressure differential situations and offers a different and better solution than proposed in that article.
With hindsight, one seeking cure to the pressure differential based deflection problem might first briefly think to make reference for same to the individual semiconductor packaging art, that is, the packaging semiconductors individually, and quickly dismiss the thought.
As is well recognized, a semiconductor chip is a physically small device, perhaps no greater in size than one-quarter to one-half inch square. One such package is known as the ceramic side brazed dual in-line package. The package, that is the base, is formed of ceramic material, is of rectangular shape, a Kovar.TM. metal seal ring forms a rectangular wall of short height on the base enclosing the semiconductor, and two rows of metal pins are brazed to the edges of the base, providing terminals for connection to other electrical circuits. The semiconductor device is installed in the package. And means are provided for connection the semiconductor device bonding pads to metal pathways incorporated into the package that connect to the metal pins. The semiconductor package is also hermetically sealed by welding or soldering a closure lid atop the seal ring. The closure lids for those packages are formed either of a compatible metal such as Kovar.TM., or of the same ceramic material that forms the package. Thus the smaller individual chips are recognized to share a generic similarity with the physically larger MCMs.
The reader is reminded that, notwithstanding the fact that the package seal ring and/or lid is formed of a metal alloy, MCM individual chip packages are generally referred to by technicians in the industry as ceramic packages, however imprecise that reference may be, so long as the package contains a base that is formed of ceramic material.
In looking to the individual chip packaging art, however, one is quickly reminded that reworkability is not a requirement for those semiconductor packages as it is for MCM. The devices cannot be reworked. If a semiconductor chip fails to perform, it is simply discarded and replaced by a like component. If it fails in operation in a circuit, the package is unsoldered from its place on the printed circuit board and is replaced with another like chip.
As stated by Byrne in U.S. Pat. No. 4,769,272, a patent that describes a ceramic package for an individual chip, and alluded to in his divisional patent U.S. Pat. No. 4,833,102, metal lids, such as nickel/iron alloy lids, can be "popped off" a ceramic package, viewed in the single chip package art as a disadvantage, but the ceramic lids cannot. To effect removal of a sealed ceramic lid, according to Byrne, the lid ordinarily must be fractured and removed in pieces. Hence, putting higher cost of the ceramic aside, the ceramic lid is believed more reliable than the Kovar.TM. for packaging individual semiconductor chips because it is permanent and cannot be reworked.
Were the lack of reworkability taught by Bryne not discouragement enough, one also finds that no one in that art appears to address a problem of excess lid deflection. As example, faced with manufacturing difficulties of high volume production, in U.S. Pat. No. 4,356,047, Gordon finds that bare wires used to bond the connection sites of a semiconductor chip to the circuit board in an individual chip package, could contact the gold plating on the Kovar.TM. lid, if misrouted during assembly, and short-circuit the chip, which must be discarded, thereby lowering production yield. Gordon proposes use of ceramic, a non-conductor, for the lid material and solders the ceramic lid to the seal ring.
Further, in U.S. Pat. No. 4,750,665, Falanga finds that a Kovar.TM. lid corrodes when exposed to salt air or other corrosive atmosphere and substitutes a ceramic sheet supported by a solder wettable metal frame that is soldered to the package's seal ring.
Then, Stradley U.S. Pat. No. 4,975,762 who employs a ceramic lid for an individual chip package, as suggested by Gordon, but incurs problems due to the existence of alpha particles generated naturally by Thorium and/or Uranium impurities within the ceramic used. The alpha particles could shower the semiconductor and cause mis-operation, referred to as "soft errors". To prevent that Gordon requires the addition of an alpha particle blocking layer to the ceramic. Stradley suggests that the cure to one problem often engenders other problems.
And then, in U.S. Pat. No. 5,495,900, Dunaway, who exposes his packaged chips to high energy particles, such as X-radiation, finds gold plated Kovar.TM. lids liberate and shower the packaged semiconductor chip with electrons, and that those X-rays can produce thermal mechanical shock of the solder bonding the ceramic lids in the package. To avoid that damage Dunaway substitutes a metal framed ceramic sheet for the Kovar.TM. lid, and connects the sheet to the frame and the frame to the seal ring with a solder free welded bond.
The prior art reviewed in hindsight addresses a litany of problems, but none appear to address a problem with excess lid deflection.
As a lay person recognizes from experience, pushing down on the middle of a metal soda pop bottle cap, while the cap is in place on the bottle, produces indiscernible deflection of the cap's surface. But, pushing down on the middle of the lid of a can of tomato's with the same force, produces discernable deflection, even though the thickness of the can's lid is greater than that of the corresponding surface of the bottle cap. The diameter of the tomato can lid being much larger, because of the greater distance of the pressing force to the supporting edges, one obtains a greater mechanical advantage and pushing down on the tomato can's lid thereby produces deflection.
Experience teaches that it is not worthwhile to try to deflect the bottle cap surface and any thought of attempting to do so no longer comes to mind. But the tomato can sometimes becomes a sort of toy noisemaker, sometimes producing a popping sound that gives momentary amusement. Those skilled in the art will recognize the same distinction between the small sized lids used in packages for individual semiconductors and the much larger lids required for large multi-chip modules, those having length or width greater than about 2 inches. The lack of relevance of the individual chip packaging structures to the deflection problem is clear. One having the deflection problem with MCM package lids, is thereby led to continue research to uncover a solution.
Accordingly, an object of the present invention is to provide a lid for large MCM's that does not significantly deflect when subjected to large differential pressures.
Another object of the invention is to provide a new lid for a MCM package that does not require a change in the physical dimensions of the MCM package and/or the addition of stiffening posts to avoid excessive lid deflection when the MCM package is subjected to large stresses created by large pressure differentials existing between the gas atmosphere internal of the package and that on the exterior.
A further object of the invention is to provide an improved multi-chip module package that withstands large pressure differentials without encountering significant elastic lid deformation and which is reworkable.